Method and apparatus for electrostatically maintaining substrate flatness

ABSTRACT

An apparatus and method for holding a substrate on a support layer in a processing chamber. The method includes the steps of positioning the substrate a predetermined distance from the support layer, introducing a plasma in the processing chamber, lowering the substrate to a point where the substrate engages the support layer, and maintaining the plasma for a predetermined time. The apparatus is directed to a susceptor system for a processing chamber in which a substrate is electrostatically held essentially flat. The apparatus includes a substrate support and a support layer composed of a dielectric material disposed on the substrate support. At least one lift pin is used for supporting the substrate relative to the support layer. Means are provided for moving each lift pin relative to the support layer. Means are also provided for producing a plasma within the processing chamber.

FIELD OF THE INVENTION

The present invention relates generally to a substrate support formaintaining an essentially flat substrate, and more particularly to ahot substrate support that electrostatically maintains the flatness of asubstrate.

BACKGROUND OF THE INVENTION

A susceptor is a mechanical part that functions as a ground electrodeand holds a substrate in a processing chamber during fabrication, suchas plasma-enhanced chemical vapor deposition (PECVD). The susceptorincludes a substrate support plate mounted on a stem, along with a liftassembly for raising and lowering the substrate within the processingchamber. The substrate is held essentially flat to facilitate thedeposition process.

The extent to which the substrate is held flat generally leads to moreuniform structural parameters across the substrate surface. For example,it is easier to grow a film of uniform thickness on a flat substratethan on one that may have a degree of curvature due to, for example,thermal stress. Thus, if uniform structural parameters are required bythe process, the substrate must be held essentially flat.

In the absence of mechanisms which physically hold the substrate flat,substrates tend to become slightly curved during processing for a numberof reasons. For example, a nonuniform temperature across the substratetends to induce a curvature due to different amounts of thermalexpansion at different areas within the substrate. In a large substrate,for example 550×650 square millimeters (mm²), a significant differencein thermal expansion may occur because the substrate heater may not beable to provide a uniform temperature across the large dimensions of thesubstrate. Further, the perimeter of the substrate has more surface areathan the central areas and thus radiates heat faster than the centralareas, again leading to temperature nonuniformity, thermal stress andconsequent curvature. In smaller substrates, for example, substratesaround 360×450 mm², the problem is less pronounced but neverthelessevident.

All of the above difficulties become worse as the processing temperaturerises. At a typical processing temperature of 320 degrees Celsius (°C.), which is common for a glass substrate, the glass substrate willlose its flatness due to the kinds of thermal stress mentioned above.

It is thus important to hold substrates essentially flat to prevent suchcurvature. Previous methods and apparatuses for holding substrates flatemploy a frame which physically contacts the substrate around thesubstrate's perimeter and holds the substrate against the substratesupport by the support's weight. Several difficulties have been noticedwith such systems.

First, the substrate area covered by the frame is sacrificed. Thus,there is less surface area of the substrate which may be used fordevices or deposition. If the entirety of the perimeter of the substrateis under the frame, substantial loss of surface area may result.

Second, the thickness of the deposited materials is not constant nearthe frame. This is primarily a geometric effect and occurs because ofthe thickness of the frame. In regions near the center of the substrate,impinging deposition gas molecules or atoms strike the surface of thesubstrate over a solid angle of 2π steradians corresponding to ahemisphere. Near the perimeter of the substrate, the frame partiallyblocks gas molecules over a significant fraction of the 2π angle. Near acorner of the frame, blockage is even worse. Thus, it is expected thatless gas molecules strike the substrate near the substrate's perimeter.As a result, the thickness of deposition is usually not uniform near theperimeter of the substrate.

Third, deposited material may seep under the frame. Such material cannotbe used in films because its thickness is uncontrollable. This problemarises because the frame typically does not contact the substrate in anabrupt manner. In other words, the effective deposition “shadow” of theframe (the point at which edge of the frame starts to inhibitdeposition) is not at the same point where the frame physically touchesthe substrate. One reason for this is that the frame may not becompletely parallel to the substrate when intimate contact is made. As aresult, some deposition may occur on the substrate under the frame. Ofcourse, the amount of such deposition is less than on the unframedcentral region of the substrate. This deposition may be problematic inthe sense that it is uncontrollable.

Fourth, a physical frame for holding the substrate flat constitutes alarge structure to be placed in a processing chamber. As such, it is apotential source for contaminant particles in the chamber which maydegrade the quality of the deposited film. This may be particularly trueas the contact between the frame and the substrate often causes particlerelease due to friction. Such particles can also adversely affect thequality of the chamber vacuum.

Fifth, a physical frame affects the reliability of transfer when asubstrate is processed in one chamber and then moved to another forfurther processing. In particular, as a substrate is transferred fromone chamber to another, a new frame is usually used. Each frame must bealigned in each processing chamber to the same position to avoid a lossof substrate processing area due to misalignment. When misalignmentoccurs, some of the substrate processing area used in one chamber isshadowed by the frame in the next chamber. Further, some of thesubstrate previously shadowed by a frame in the one chamber is notcovered in the next chamber. In both cases, these areas must besacrificed as not having been fully processed. To combat this problem,complicated realignment mechanisms must be used to ensure the same areais covered by each frame. Such mechanisms again lead to moreparticle-releasing surface area in the chamber and ensuing particlecontamination and breakdown. Such mechanisms are also expensive andcomplex, increasing markedly the manufacturing cost of the processingchamber.

The inventors have discovered a need to provide a method and apparatusfor keeping substrates essentially flat to increase the usable substratearea and to enhance film uniformity across this area, particularly nearthe edges of the substrate. The method and apparatus should not requirecomplex mechanisms, and should not lead to contamination of theprocessing chamber. The present invention fulfills these needs.

SUMMARY

In one embodiment, the invention is directed to a method for holding asubstrate on a support layer in a processing chamber. The methodincludes steps of locating the substrate a predetermined distance fromthe support layer, starting a plasma in the processing chamber, loweringthe substrate to a point where the substrate engages the support layer,and maintaining the plasma for a predetermined time.

Implementations of the invention may include one or more of thefollowing. The method may further comprise steps of stopping the plasmaand depositing a film on the substrate. The plasma may constitute a gasthat is inert to the substrate, for example one selected from the groupconsisting of nitrogen, hydrogen, argon, helium, krypton, xenon, neon,radon, mixtures thereof, or other similar gases, molecular or otherwisethat can form a plasma. The pressure of the gas may be in a range offrom about 200 mTorr to about 1 Torr. The power of the plasma may be ina range of from about 100 watts to about 1000 watts. The power densityof the plasma may be in a range of from about 0.02 watts per squarecentimeter of substrate area to about 0.5 watts per square centimeter ofsubstrate area, or about 0.4 watts per cubic centimeter of chambervolume to about 4 watts per cubic centimeter of chamber volume. Thesubstrate may be made of glass. The support layer preferably is adielectric material, such as anodized aluminum or alumina (Al₂O₃). Themethod may further comprise the step of depositing a coating on top ofthe support layer. The preferred coating may be selected from the groupconsisting of silicon nitrides, silicon oxides, silicon carbides andmixtures thereof.

In another embodiment, the invention is directed to a susceptor systemfor a processing chamber in which a substrate is electrostatically heldessentially flat. The system includes a substrate support and a supportlayer composed of a dielectric material disposed on the substratesupport. At least one lift pin is used for supporting the substraterelative to the support layer. Means are provided for moving each liftpin relative to the support layer. Means are also provided for byigniting a plasma within the processing chamber. A gas supply supplies agas to the interior of the processing chamber.

In another embodiment, the invention is directed to a method forprocessing a substrate in a processing chamber. The method includessteps of locating the substrate a predetermined distance from a supportlayer, starting a plasma in the processing chamber, lowering thesubstrate to a point where the substrate engages the support layer,maintaining the plasma for a predetermined time, and depositing orgrowing a thin film on the substrate.

Among the advantages of the invention are the following. A method andapparatus are provided for maintaining a substrate in an essentiallyflat position. An increase in usable substrate area is achieved, andfilm uniformity grown thereon is enhanced. The method and apparatus donot require complicated frame mechanisms, and thus have the advantage ofnot locating potentially contaminant-producing structures into theprocessing chamber. The method and apparatus may be used in processingsubstrates in semiconductor processing chambers.

The details of one or more embodiments of the invention are set forth inthe accompanying drawings and the description below. Other features,objects, and advantages of the invention will be apparent from thedescription and drawings, and from the claims.

BRIEF DESCRIPTION OF THE DRAWINGS

Like reference numbers and designations in the various drawings indicatelike elements.

FIG. 1 is a cross-sectional view of a CVD processing chamber.

FIG. 2 shows the electrical connections to a heating element used in theCVD processing chamber of FIG. 1.

FIG. 3 is a top view of a substrate support plate used in the CVDprocessing chamber of FIG. 1.

FIG. 4 is a cross-sectional view of a processing chamber used accordingto the present invention showing details of the substrate support andthe plasma used in the CVD processing chamber of FIG. 1.

DETAILED DESCRIPTION

The present invention is directed to a method and apparatus formaintaining a substrate essentially flat. In the implementationdescribed below, the invention is described with respect to a CVDchamber. However, the invention is also applicable to other sorts ofprocessing chambers. For example, the invention may be used in chamberswhich carry out the following fabrication processes: CVD, PECVD, etchingprocesses, physical vapor deposition (PVD), and rapid thermal processessuch as rapid thermal annealing (RTA). Certain of the details describedare specific to this implementation and may be changed as required bythe processing conditions and parameters.

The present invention may be used in a model AKT-3500 PECVD System,manufactured by Applied Komatsu Technology of Santa Clara, Calif. TheAKT-3500 PECVD is designed for use in the production of substrates forlarge liquid crystal flat panel displays. It is a modular system withmultiple process chambers which can be used for depositing amorphoussilicon, silicon nitrides, silicon oxides, and oxynitride films. Moredetails regarding the system may be found in U.S. patent applicationSer. No. 08/707,491, entitled “A Deposition Chamber Cleaning TechniqueUsing a High Power Remote Excitation Source”, filed Sep. 16, 1996,assigned to the assignee of the present invention and incorporatedherein by reference. The present invention, however, may be used withany commercially-available deposition system.

PECVD or CVD are processes used to deposit a thin film layer onto asubstrate. We intend the term “substrate” to broadly cover any objectthat is being processed in a process chamber. The term “substrate”includes, for example, flat panel displays, and glass or ceramic platesor disks. The present invention is particularly applicable to largesubstrates such as glass plates having areas of 360×450 mm², 550×650mm², and larger. The remainder of this detailed description describes anembodiment in which a glass substrate is used. However as noted above,other substrates may also be used.

In general, the substrate is supported in a vacuum deposition processchamber and is heated to several hundred ° C. Deposition gases areinjected into the chamber, and a chemical reaction occurs to deposit athin film layer onto the substrate. The thin film deposited layer may bea dielectric layer (such as silicon nitride or silicon oxide), asemiconductor layer (such as amorphous silicon), or a metal layer (suchas tungsten). The deposition process may be PECVD or thermally-enhancedchemical vapor deposition. In the chamber shown in FIG. 1, a plasma isused. Thus, appropriate plasma ignition means, such as the radiofrequency (RF) voltage described below, are generally required.

As shown in FIG. 1, a CVD apparatus 130 includes a susceptor 135 havinga substrate support plate 20 mounted on a stem 137. Support plate 20 maybe fabricated, for example, of high purity unanodized cast aluminum orof aluminum alloys. Susceptor 135 is shown centered within a vacuumdeposition process chamber 133. A support layer 22 is located on a platesurface 176 of support plate 20 to support a substrate such as a glasspanel (shown in FIG. 4) in a substrate processing or reaction region141. As discussed in more detail below, and in accordance with thepresent invention, support layer 22 constitutes a dielectric material. Alift mechanism (not shown) is provided to raise and lower the susceptor135. Commands to the lift mechanism are provided by a controller inknown fashion. Substrates are transferred into and out of chamber 133through an opening 142 in a sidewall 134 of the chamber 133 by a robotblade (not shown).

The deposition process gases (indicated by arrow 123) flow into chamber133 through an inlet manifold 126. The gases then flow through aperforated blocker plate 124 and holes 121 in a process gas distributionfaceplate 122 (indicated by small arrows in the substrate processingregion 141 of FIG. 1). Support layer 22 of support plate 20 is paralleland spaced-closely to faceplate 122. An RF power supply 172 (shown inFIG. 4) may be used to apply electrical power between gas distributionfaceplate 122 and susceptor 135 so as to excite the process gas mixtureto form a plasma. The constituents of the plasma react to deposit adesired film on the surface of the substrate on support plate 20.

The deposition process gases may be exhausted from the chamber through aslot-shaped orifice 131 surrounding the reaction region 141 into anexhaust plenum 150. From exhaust plenum 150, the gases flow by a vacuumshut-off valve 152 and into an exhaust outlet 154 which connects to anexternal vacuum pump (not shown).

Referring to FIG. 2, support plate 20, as noted, is attached to stem 137of susceptor 135. Support plate 20 may include a top plate 40, a baseplate 42, and a braised region 44 therebetween. Disposed in supportplate 20 between top plate 40 and base plate 42 are heaters 24 and 26.In the illustrated embodiment, one or more heating element tubes 50(only one is visible in FIG. 2) are disposed within the hollow core ofstem 137. Each tube includes a conductive lead wire 52 for attachment toan end of a filament of a heating element. Tubes 50 are terminated atthe end of stem 137, and lead wires 52 are connected to a heatercontroller which powers the heating elements and monitors thetemperature of support plate 20. The heaters may be located about 0.25in. beneath the uppermost surface of support layer 22 on support plate20. In this embodiment, one heater is run at approximately 0.66 in. fromthe outer edge of the plate, while another is run at approximately 7.75in. from the outer edge. This configuration provides for uniform heatingof a substrate 165 placed on support plate 20.

Stem 137 includes a hollow core and is configured to mate with baseplate 42 of support plate 20. A vacuum tight joint 85 is made such thatthe inside of the hollow core is at ambient (atmospheric) pressure.

FIG. 3 shows a top view of substrate support plate 20. As noted, supportplate 20 includes a support layer 22 for supporting a substrate 165(shown in phantom) in the vacuum deposition process chamber. Heaters 24and 26 (both shown in phantom) are disposed beneath support layer 22 onsupport plate 20. Support plate 20 is rectangular in shape, and has awidth “w” of about 26.26 inches and a length “1” of about 32.26 inches.

FIG. 4 shows the depth “d” and length of support plate 20. A typicaldepth of support plate 20 may be about one inch. This allows for theprocessing of a glass substrate for flat panel displays of up to about 1square meter (m²). The size of support plate 20 is scalable toaccommodate either larger or smaller substrates.

Support layer 22 is disposed on top of support plate 20. Support layer22 may be a separate plate placed in intimate contact with a surface 176of support plate 20 or a thick layer of a material coated on surface 176of support plate 20. The material constituting support layer 22 isanodized or otherwise treated so that support layer 22 has theproperties of a dielectric. For example, anodized aluminum or alumina(Al₂O₃) may be used. In this way, any charge induced on a surface 23 ofsupport layer 22 is stationary.

A coating (not shown) may be deposited on surface 23 of support layer 22to enhance the dielectric properties of support layer 22. In otherwords, if support layer 22 by itself is not a superior dielectric, thecoating may improve its dielectric properties. Such coatings mayconstitute dielectrics, and may be, for example, silicon nitrides(Si_(x)N_(y)), silicon oxides (Si_(w)O_(z)), silicon carbides(Si_(r)C_(s)) or other such dielectrics. One type of SiN deposition isdescribed in U.S. Pat. No. 5,399,387, assigned to the assignee of thepresent invention and incorporated herein by reference. This coatingenhances the effect of the invention but is not inherently required forits practice.

As mentioned above, a robot blade facilitates the transfer of substratesinto and out of chamber 133 through an opening 142 in sidewall 134 ofchamber 133. Once the robot blade moves substrate 165 into position,lift pins 171 move upwards to support substrate 165 prior to loweringinto a processing position. In particular, lift pins 171 move throughlift pin holes 162 to contact and support substrate 165. Lift pins 171may move through lift pin holes 162 by the action of a lift means 180such as known translation mechanisms or linear feedthroughs.

After substrate 165 has been contacted and supported by lift pins 171,the robot blade is withdrawn and substrate 165 may be lowered intoposition for processing. In the method of the invention, substrate 165is not lowered into intimate contact with support layer 22 (or acoating) until after a plasma 169 is ignited in the chamber.

In particular, lift pins 171 retract and lower substrate 165 until abottom surface 173 of substrate 165 is at a predetermined separationdistance, in a range of about 20 to 50 mils above surface 23 of supportlayer 22. This position is termed here an “intermediate height”. Liftpins 171 may be attached to a moving means which is remotely and/orcomputer-controlled by a controller 177. At this point, a plate-chargeinducing plasma 169 is started or ignited in chamber 133. The separationdistance is chosen to be wide enough for this plasma to enter the volumebetween substrate 165 and support layer 22. If the distance is toosmall, no plasma will enter this volume and no charge effects willoccur. If the distance is too large, the plasma may become unstable nearthe edges of substrate 165.

Plate-charge inducing plasma 169 is formed from a gas that is relativelyinert to the substrate, such as nitrogen (N₂), hydrogen (H₂), argon(Ar), helium (He), krypton (Kr), xenon (Xe), radon (Rn), or mixturesthereof. Other gases with similar plasma properties may also be used. Arequirement of the gas used is that it not itself deposit on substrate165. The power of plasma 169 may be relatively low, such as in a rangeof about 100 watts to 1000 watts for a chamber having a volume of 250cubic centimeters and a substrate processing area of 550×650 mm²; thepower would scale up or down for chambers of larger or smaller volumes,or larger or smaller substrate processing areas, respectively. Forexample, as the power scales with the size of substrate 165, a usablerange of power densities may be 0.02 watts per square centimeter to 0.5watts per square centimeter of substrate area. As the power also scaleswith the volume of chamber 133, a usable range of power densities may be0.4 watts per cubic centimeter to 4 watts per cubic centimeter ofchamber volume.

The pressure of plate-charge inducing plasma 169 may be between about200 milliTorr (mTorr) and 3 Torr. Generally lower pressures arepreferred because they allow a larger bias to be induced betweensubstrate 165 and support layer 22 (in a manner to be explained below).This bias refers to the amount of charge induced on the bottom surface173 of substrate 165 and the amount of charged induced on support layer22. These charges are responsible for an electric field having featuresand advantages discussed below.

After plasma 169 is ignited, substrate 165 is lowered onto support layer22, to a “processing height”, by retracting lift pins 171. The rate atwhich the substrate is dropped may be on the order of an inch every fewseconds, such as one inch every three seconds. After placement ofsubstrate 165 on support layer 22, plasma 169 is maintained for apredetermined time which may be measured in seconds. For example, tenseconds has been found to be appropriate for a large glass substrate.However, it should be noted that there is a wide range of variation inthe length of this time period. Longer or shorter time periods may alsobe used depending on the substrate material, the plasma power, thesupport layer material, the coating material, and the type of gas usedin the plasma.

After engaging substrate 165 on support layer 22 and maintaining plasma169 for several seconds, substrate 165 is held in an essentially flatmanner against support layer 22 and is highly resistant to curvature.

One reason why substrate 165 is held to support layer 22 is believed tobe because of electrostatic attraction. In particular, it is noted thatplate-charge inducing plasma 169 is composed of electrons (denoted inFIG. 4 as “−e”) and positive ions (denoted in FIG. 4 as “+i”). Thevoltage applied via voltage source 172 to ignite plasma 169 is an RFvoltage which alternates between a positive value and a negative value.The potential of susceptor 135 is held to ground. When the appliedvoltage is positive, electrons are attracted to faceplate 122. When theapplied voltage is negative, positive ions are attracted to faceplate122. As even the smallest positive ion is about 2000 times more massivethan an electron, the electrons move much faster due to lower inertia.When the electrons are attracted to the faceplate 122, a net positivecharge is left in plasma 169 because most of the remaining specieswithin plasma 169 are positive ions. Even when electrons are repelled bya negative voltage swing of source 172, they are not believed to berepelled so far as to render the net charge of plasma 169 neutral. Thisis indicated in FIG. 4 by the placement of several electrons adjacentfaceplate 122. The net positive charge of plasma 169 (indicated in FIG.4 by a preponderance of positive ions in plasma 169) then induces anegative charge on a top surface 175 of substrate 165 (indicated by aseries of negative signs near surface 175). By conservation of charge,this induced negative charge in turn induces a positive charge on bottomsurface 173 of substrate 165 (indicated by a series of positive signs onsurface 173). This positive charge then induces a negative charge on topsurface 23 of support layer 22 (indicated by a series of negative signson surface 23).

Another factor enhances the induced negative charge on top surface 23.Because surface 23 is not completely electrically screened from plasma169 (such partial screening occurring because of the placement ofsubstrate 165), the net positive charge of plasma 169 also tends toinduce a negative charge on surface 23 of support layer 22. Thus, anelectrostatic attraction is formed between surface 23 of support layer22 and bottom surface 173 of substrate 165.

To summarize, bottom surface 173 of substrate 165 is left with aninduced positive charge, and top surface 23 of support layer 22 is leftwith an induced negative charge. By Coulomb electrostatic attraction,substrate 165 is held substantially flat against support layer 22.

Following several seconds of plasma 169, plasma 169 may be stopped orextinguished and further processing of substrate 165 may begin. Furtherprocessing may also begin without stopping plasma 169. In this method,the inert gas or gases forming plasma 169 are simply replaced byappropriate process gases while source 172 continually causes the gasesto enter the plasma state.

Further processing may include film deposition. Such processing mayinvolve reinstatement of a different plasma for use in deposition orother processes, and this plasma may be of an inert gas or otherwise.

Once further processing is completed, substrate 165 is removed fromsupport layer 22. This may be accomplished by using lift pins 171 toforce the substrate off of support layer 22. Another way to removesubstrate 165 from support layer 22 is described in U.S. Pat. No.5,380,566, issued Jan. 10, 1995, assigned to the assignee of the presentinvention and incorporated herein by reference.

In summary, a substrate support including an electrostatic substrateattachment feature has been disclosed. This feature allows a substrateto be held essentially flat against a support layer without loss ofusable substrate area.

A number of embodiments of the present invention have been described.Nevertheless, it will be understood that various modifications may bemade without departing from the spirit and scope of the invention.

What is claimed is:
 1. A method of holding a substrate on a supportlayer in a processing chamber, comprising the steps of: positioning asubstrate a predetermined distance from the support layer; introducing agas into the chamber wherein the gas is inert relative to the substrate;igniting the gas under conditions so as to form a plasma in the chamber,thereby creating a positive charge on a surface of the substrate facingthe support layer and a negative charge on a surface of the supportlayer facing the substrate; maintaining the positive and negativecharges over a period of time thereby holding the substrate in a desiredposition relative to the support.
 2. The method of claim 1, furthercomprising: depositing a coating on the surface of the support layerfacing the substrate layer.
 3. The method of claim 2, wherein thecoating is composed of a material selected from the group consisting ofsilicon nitrides, silicon oxides, silicon carbides and combinationsthereof.
 4. The method of claim 2, further comprising: subjecting asurface of the substrate facing away from the support layer to furtherprocessing.
 5. The method of claim 4, wherein the further processingforms a layer on the substrate surface.
 6. The method of claim 5,wherein the further processing is selected from the group consisting ofchemical vapor deposition and physical vapor deposition.
 7. The methodof claim 1, wherein the inert gas is selected from the group consistingof nitrogen, hydrogen, argon, helium, or mixtures thereof.
 8. The methodof claim 1, wherein the conditions comprise a pressure of the gas is ina range of from about 200 mTorr to about 1 Torr.
 9. The method of claim1, wherein the conditions comprise applying power in range from about 10watts to about 1000 watts.
 10. The method of claim 1, wherein theconditions comprise maintaining a power density in a range from about0.02 watts per square centimeter of substrate area to about 0.5 wattsper square centimeter of substrate area.
 11. The method of claim 1,wherein the conditions comprise maintaining a power density in a rangefrom about 0.4 watts per cubic centimeter of chamber volume to about 4watts per cubic centimeter of chamber volume.
 12. The method of claim 1,wherein the substrate is comprised of glass.
 13. The method of claim 1,wherein the substrate is comprised of ceramic.
 14. The method of claim1, wherein the support layer is comprised of a dielectric material. 15.The method of claim 14, wherein the dielectric material is comprised ofanodized aluminum.
 16. The method of claim 14, wherein the dielectricmaterial is comprised of alumina (Al₂O₃).